1. Field of the Invention
The present invention relates to a device for use in testing visual acuity contrast testing, glare testing, duochrome testing, and amblyopia testing.
2. Description of the Prior Art
The Snellen chart is the most widely used method for testing of visual acuity. Originally, a wall chart having a number of rows of characters of different sizes was employed. Subsequently, the projector chart was introduced, in which a single line of characters of a predetermined size is projected onto a screen. Testing is typically accomplished by having the patient view the chart through a phoropter, i.e., an optical device containing numerous lenses which may be selectively introduced into the line of vision of the patient. While the patient is viewing the projected eye chart, the phoropter controls are manipulated until a lens combination which achieves optimum vision is obtained. Whenever it is desired to change the size of the projected characters, the examiner must move away from the phoropter to operate the projector.
A major drawback of projector testing stems from the fact that the characters which are projected are fixed. The lenses of the phoropter are typically altered while having the patient view the minimum visible line in order to achieve refraction, i.e., selection of the proper lens combination for the patient. The repeated viewing of the same line leads to problems of memorization of the characters. The patient cannot subjectively distinguish between better vision resulting from a new lens combination and knowledge of the letters.
Acuity testing is typically accomplished in a small room, with the projector positioned next to the examiner and multiple mirrored paths of light being employed to achieve the necessary projection distance (approximately twenty feet) so that the light rays reaching the patient may be considered parallel. Because of the multiple mirrored paths of light, the examiner or other personnel in the room may get in the projection path. This is especially so in the case of the examiner, since the examiner must turn to the projector every time letter size is to be changed and then return to manipulate the phoropter dials.
In an attempt to overcome some of the disadvantages of the use of a projection chart for refraction, the system disclosed in U.S. Pat. No. 4,239,351 to Williams et al employs a video display for the purpose of displaying visual acuity targets to be viewed by the patient. An operator's control console contains control buttons for selecting the size of the targets to be displayed. The control console operates to generate letters in a random fashion, thus avoiding the problem of patient memorization of test letters. The control console includes its own video display, thus enabling the examiner to face in the general direction of the patient while selecting the targets to be generated and avoiding the need for the examiner to turn to see the targets displayed on the patient display unit. The video display unit of the control console is substantially the same size as that of the patient display unit and is in fact bulkier than the typical prior art projector unit. Although multiple mirrored projection paths are eliminated, the testing procedure is still awkward and inefficient, since the examiner must move away from the phoropter dials to the control console to select a new display.
Several features not possible with a standard projector are disclosed in the above-mentioned U.S. Pat. No. 4,239,351. These include the ability to project white targets on a black background, a zoom capability for continuously altering the size of the displayed targets, and means for controlling ambient light with respect to the patient display to enable acuity to be tested at more norma-1 levels of ambient illumination. However, the basic testing procedure is essentially identical to that employed with a standard projection system. Furthermore, target generation on the video display is accomplished by means of block graphics, in which each target is described by a 5.times.5 matrix of light and dark areas. The matrix is stored in memory and acted upon by an appropriate scale factor to achieve the desired target size. The use of block graphics limits the resolution of the system. In addition, it limits the types to targets which can be generated to those which may be defined in terms of blocks in the matrix.